


He's Oblivious

by flustered_mugs



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, Fantasizing, Implied/Referenced Incest, Incest, M/M, Masturbation, Original Character(s), Sibling Incest, Underswap Papyrus, Underswap Sans, Voyeurism
Language: English
Status: Completed
Published: 2018-06-11
Updated: 2018-06-11
Packaged: 2019-05-20 22:23:06
Rating: Not Rated
Warnings: Major Character Death
Chapters: 1
Words: 2,147
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14903205
Author URL: https://archiveofourown.org/users/flustered_mugs/pseuds/flustered_mugs
Summary: Sans tries to ignore what’s going on in his brother’s room. He’s not very good at accomplishing that.





	He's Oblivious

It's incredible that Papyrus' first impression of humans has to be that of a sexual nature; with the fallen human taking what Papyrus had mistaken for a date proposal and turning it into something much, much more vulgar.

Sans is the one who encouraged Papyrus with his desire to meet the human after all- his smile kind and warm as he had watched his younger brother's initial reaction to having seen the human for the first time. The sight of one isn't anything new to Sans himself, he's seen plenty of photographs of them in Human biology textbooks that he'd find in the Waterfall dump. There was never any real interest that he held for humans, so he always took care to take the textbooks out of the pile of trash and set it aside to where the rest of the novels were stacked up, curtosy of monsters helping other monsters find new reading material from above ground.

But it was funny at first to see just how much emotion this simple human was able to draw out of his brother, even when he's gotten a little more reclusive with his emotions in the most recent years. 

Now though, Sans can't help but wish that he had leaned a little more towards being defensive against the human, rather than supportive.

He's lying on his uncovered mattress, eyes boring into the ceiling of his bedroom as he keeps his fingers intertwined over his abdomen, only for him to switch the position out so that he can wring his fingers into the cotton of his shirt as he tries to tune out the muffled noise coming from within Papyrus' bedroom.

"Ha....ha...." There's a strain to the noises that leave Papyrus, his voice higher than normal and many times more expressive. "Ahh..." The noise draws out, hitching at the end as he goes silent for a few moments and Sans can hear the telltale slap of skin on magic-made "skin", and he has to close his eyes and take a few deep breaths when he realizes just how wet it all sounds.

He tries to ignore the blue light from below illuminating his face, refusing to move his head from where he faces the ceiling. But the thrumming of magic in his system proves just a bit harder to pretend it doesn't exist, not when he can feel it vibrating and thrumming throughout his body like alcohol, leaving him just as warm and light headed as he would be when intoxicated. Because that's what all of this really is; intoxicating.

His trademark blue bandanna feels tighter than ever around his throat.

When he reaches up to loosen the restriction around his neck, the effort he has to put into the action has his arm feeling full of cotton, moving slowly and stiffly. But he gets it done just in time to hear a particularly delicious noise from his brother's room that so obviously didn't come from the human themselves but has Sans clenching his eyes shut again and throbbing in his pants anyways.

He wishes he could tell himself that he's only half hard, or that his magic hasn't even fully formed his dick yet- but that'd be a big, fat lie. He's got to face the big fat truth instead, while it's straining at the front of his slacks and begging for some tender loving.

This whole time has been a struggle for Sans to ignore the ridiculously vulgar noises leaving the mouth of a monster he shares a kinship with- for God's sake- and ignore the lively levels of activity coming from his magic as it pools into the lower regions of his body, regions he's trying so, so hard to pretend don't exist right now.

But then he hears it. 

The faintest huff coming from a voice too different to his brother's, softer, more reserved.

It comes again, and again, their breathing heavy enough that he can hear it through the wall separating his bedroom from Papyrus', and the the childish racecar bed (that they defile together) is groaning and squeaking just as much as his brother is at this point.

They must be close. 

Swallowing hard, Sans raises a shaky hand and slowly traces it down his stomach, further down into the brim of his slacks until he's touching neither cotton nor bone, but monster magic.

It's because of the human that he's touching himself. 

"W-Wait, stop. My brother will h-h-...ah..." 

He grits his teeth and exhales sharply through his nasal cavity, unintentionally gripping his dripping arousal much tighter than he intended, but even then he's still not going to let go.

It's hard and his movements are jittery, and funnily enough he's aware of it enough to be reminded of the time he had underestimated the effect a single cup of coffee would have on him and he wants to focus on that and hope that it helps flag his arousal, but it doesn't, so he starts stroking anyways. Slow and clumsy.

It's the human, he's listening to the human.

"Rrgh! O-Oh God!" Every single one of Papyrus' gasps and moans are punched out of him as the human thrusts into him with wreckless abandon, the noise of their love-making (he shouldn't lie to himself, they're fucking) loud and clear even through the wall.

Breathing isn't as easy as it used to be anymore and staying quiet is a chore, but Sans does his best to devote himself to it, the entire frame of his body trembling as he arches his back and releases his dick- as stocky as himself- and palms at it like a desperate animal.

He tells himself that it's the human he's masturbating to, that he's attracted to their foreign nature and just how alien they really are to monsters such as himself.

But it's the long, lanky fit of his brother's body that occupies the imagery in his mind; the way his orange hoodie must be pushed up and over his rib cage, almost touching his chin (because that how Sans would want it, there's something just so sexy about fucking without fully undressing) and the way he's probably got his face tucked into the inside of his elbow, his legs wrapped around their (his) waist and how their flushed, red dick (his raspberry blue dick) tugs at his insides with every pull outwards, and how perfectly snug he must feel to fit inside of.

The heat overtaking his face is unbearable and he wants to undress and get better comfortable for this, but he can't and he won't. It's the blanket that goes instead, which he kicks and lets fall over the edge of his mattress. He doesn't care anymore that he can see the prominent movement of his hand beneath his slacks, running the length of his cock up and down several times.

Papyrus isn't talking anymore, but he's breathing, by God he's breathing like he's drowning and Sans can tell the tempo they're going at because there's a hitch in his breath for each time he gets fucked into, and it's funny that it's that he focuses on, as if the sound of skin-on-skin wasn't enough to tell.

There's a soft whispering that Sans can't make out, but whatever the human says must be pretty bad because Papyrus is groaning next and using what little coherent thought he has left to try and talk.

"N-No, he'll....Ha-....Sans would hear..." His voice is shot, throat croaky like he's spent the whole night getting fucked rather than just the past hour.

It's barely audible but Sans can hear the human ask "Who?" and he'd feel insulted if it weren't for the fact that it has his brother trying to repeat his name, and when he fails to get it right because of the way he's getting fucked into he tries again.

It's delicious.

It's so fucking good and even when Papyrus loses the ability to talk and starts to gasp and moan and reach his end's limit, Sans replays the word over and over in his mind.

"San- ngh! S...Sans...!" His brain echoes, sweat dotting his forehead as he lifts his hips up and off of the bed, finally taking himself in hand again and stroking himself quickly, violently. He thrusts up into his hand a few times in the cheap imiatation of what he'd fuck like but forces himself to still and fall back onto his mattress when the action makes his the floor squeak in response.

He's so close it hurts and the coarse material of the gloves that he didn't even bother to take off makes it so much better and he feels fucking disgusting, touching himself to the sound of his younger brother's own defilement, to the fact that his brother may be losing his virginity for the first time, to a human he just met and it's like he's just letting himself get used. Like all it took for him was a little bit of sweet talking and the excitement of engaging with something foreign, exotic, and that's enough to get him to offer his sweet insides to be played with. 

"Oh fuck! Fuck!"

The cry startles him with a jolt and his eyes snap open, his free hand coming up to cover his mouth. It must have been him who said that, and god won't Papyrus throw a fit over the fact that his own brother is speaking with such a dirty tongue, but then-

"Fuck, I'm coming, I'm-! Please, please, please!"

It's Papyrus. He's the one who said it. 

It's a bit silly for him to think that his brother- who's well into adulthood and legally allowed to buy his own house- wouldn't use language like that (or even know a thing about sex) but Sans very well believed it. He's not sure why. Maybe the image of his brother as that sweet little innocent skeleton he once was is forever imprinted on his mind, what he'll always see when he looks at him. Even when the monster towers well above him in height.

But he's cussing, and he's letting a human play with his insides, and he's probably going to let them fill him with a thick cream that his body will know exactly what to do with and he's going to touch himself during it and he'll cum to his own degradation, and-

Sans is lucky that he left his hand over his mouth. He bites into his knuckles, protected only by his glove, as his cock throbs in tune with his wildly beating soul, cum landing on the custom-fitted fabric of his royal guard uniform gloves and it takes everything he has to not scream at his white-knuckle release.

He thinks he's done after a moment or so and he lowers the hand over his mouth, moving to let go of himself, but it's with a gasp that he realizes he's still cumming. Acting on impulse in a mindless desire to draw out his own pleasure, Sans flips over onto his side, bringing his pillow between his legs and holding it tightly against his groin. He frots against the plush give of his pillow several times before, finally, his dick gives one last twitch and the contact brings him more pain than it does pleasure.

Even though he's done his magic still lingers, dimming and glowing in tune with the pulsing in his head. It's only after he licks at the roof of his mouth and lets his tongue loll out as he breathes heavily that his tongue's existence occurrs to him. He licks his teeth experimentally before groaning quietly and burying his face into his mattress.

He's a mess. 

Literally and metaphorically.

But he can't get out now and take a shower, not when he knows Papyrus and his little human..."friend" will want to use it soon, so he inhales deeply and decides to just deal with it. It's the least he deserves and the weakest punishment he can endure after what he just did. 

Within the next room, the human kisses up Papyrus' neck, suckling hickies that they don't bother to question as they train their eyes on the room they know lies hidden behind Papyrus' wall. The skeleton below them is entirely receptive, shivering and still trembling from the aftershocks of his orgasm and it makes them shudder too, to feel the tight grip on their cock loosen and tighten in tune with Papyrus' lingering orgasm. 

They came down here for a reason, and they might not have gotten what they expected, but this? This is better.

And they know a way to make it even more enjoyable.

It's sick, and maybe they'll have to force it-

They're not sure which brother is the protective one. Which one holds more morals.

But they want it, and they're going to get it.

**Author's Note:**

> hope y'all enjoyed, i was thinking of maybe continuing this but i'm not sure??? if you guys want a continuation ill go for it. sorry for any mistakes i just wanted to write something quick


End file.
